Field of the Invention
In the field of integrated circuits, output circuits exist which frequently consume a certain amount of power, such as for example, Darlington type circuits. The present invention relates to a control circuit allowing to reduce this power consumption.
Darlington type circuits are represented in FIG. 1 in several of their possible configurations. One general characteristic of Darlington type circuits is that they comprise at least two bipolar transistors and that the gain of the circuit assembly is the product of the gains of each of the transistors.
In FIG. 1A, the Darlington type circuit is constituted by a main transistor T1 of the NPN type and a control transistor T2 of the PNP type. The collector of the transistor T1 is connected to the power supply and its emitter to the earth through the intermediary of a load 10. The base of the transistor T1 is connected to the collector of the transistor T2 of which the emitter is connected to the power supply V.sub.CC. The control signal is applied to the base of the transistor T2 and is symbolized by a current supply source I.sub.B2 connected between this base and the earth.
In FIG. 1B, the Darlington type circuit comprises two PNP type bipolar transistors T1 and T2. The emitter of the main transistor T1 is connected to the power supply V.sub.CC and the collector of the transistor T1 is connected to the earth through the intermediary of a load 10. The base of the transistor T1 is connected to the emitter of the control transistor T2 of which the collector is connected to the collector of the transistor T1. The base of the transistor T2 receives the control signal schematized in the form of a current supply I.sub.B2 connected between this base and the earth.
FIG. 1C again represents a Darlington assembly in which the two transistors are of the NPN type. The collector of the transistor T1 is connected to the collector of the transistor T2, these two collectors being connected to the power supply V.sub.CC through the intermediary of a load 12. The emitter of the transistor T1 is connected to the earth and its base is connected to the emitter of the transistor T2. The base of the transistor T2 receives the control signal symbolized by a current supply I.sub.B2 connected between the power supply V.sub.CC and this base.
Thus, a Darlington type circuit can be defined in a general manner as a bipolar circuit comprising a first main transistor T1 of which a first main electrode is connected to a power supply terminal V.sub.CC and a second main electrode is connected to an earth terminal, a load 10, 12 being connected between one of these main electrodes and the corresponding terminal, the base of this first transistor being connected to a second main electrode of a second transistor, called auxiliary or control transistor, a main electrode of the second transistor being connected to a main electrode of the first transistor and the base of the second transistor receiving a control signal.
In order for a Darlington type circuit to operate satisfactorily, whether the load is connected to the earth or to the power voltage and whether the output transistor is of the PNP or NPN type, it is necessary to supply a strong base current to the main transistor in order to obtain suitable saturation. This means that, despite the gain of the auxiliary transistor T2, this auxiliary transistor must receive a high base current on the scale of currents generally present in an integrated circuit.
The power P.sub.d dissipated throughout this Darlington assembly can be expressed by the following formula: EQU P.sub.d (dissipated power)=(V.sub.CE2 +V.sub.BE1) I.sub.O +I.sub.B2 .times.V.sub.CC ( 1)
V.sub.CE2 is the collector-emitter saturation voltage of the transistor T2, PA1 V.sub.BE1 is the base-emitter voltage of the transistor T1, PA1 I.sub.O is the current in the charge, PA1 V.sub.CC is the power supply voltage of the circuit, PA1 I.sub.B2 is the base current of the transistor T2. PA1 V.sub.BE1 is the base-emitter voltage of the first transistor; PA1 V.sub.CE2 is the collector-emitter voltage of the second transistor at saturation and; PA1 V is the potential of the earth if the load is connected to the power supply terminal and is equal to V.sub.CC -V.sub.CE1 if the load is connected to the earth, V.sub.CE1 being the collector-emitter voltage at saturation of the first transistor.
As will be seen herein-below, the term I.sub.B2 .times.V.sub.CC is normally insignicant.
In order to reduce the dissipated power, configurations of the type such as represented in FIG. 2 have sometimes been adopted, that are known as "pseudo-Darlington type" circuits. These circuits are characterized by the fact that the main electrode of the control transistor T2 not connected to the base of the transistor T1, instead of being connected to a main electrode of this transistor T1 is connected to the earth or to the power voltage according to the type of bipolar transistor, through the intermediary of a resistance.
More particularly, in FIG. 2A, the two bipolar transistors of the pseudo-Darlington assembly are of the PNP type. The emitter of the transistor T1 is connected to the power supply V.sub.CC and its collector to the earth through the intermediary of a load 10. The base of the transistor T1 is connected to the emitter of the transistor T2 of which the collector is connected to the earth through the intermediary of a resistance. In FIG. 2B, the two transistors are of the NPN type. The collector of the transistor T1 is connected to the power supply V.sub.CC through the intermediary of a charge 12 and its emitter to the earth. The base of the transistor T1 is connected to the emitter of the transistor T2 of which the collector is connected to the power supply source through the intermediary of a resistance.
It can be seen that in this type of assembly the dissipated power will be substantially: EQU P.sub.d =V.sub.CE1 .times.I.sub.O +V.sub.CC .times.I.sub.B1 +V.sub.CC .times.I.sub.B2 ( 2)
where I.sub.B1 is the current base of the transistor T1, the other notations being the same as those previously indicated herein-above (the term V.sub.CC .times.I.sub.B2 is insignificant).
A numerical application allows to render more apparent the problem involved. In the case of FIGS. 1 or 2, I.sub.O =2 amperes is common (it is the high current that is sought to pass into the load), V.sub.CC =30 volts, V.sub.BE1 (for I.sub.O =2 amperes) of about 1.2 V, V.sub.CE2 (for a collector current of 50 mA) of about 0.3 V and V.sub.CE1 =0.8 V for I.sub.O =2 amperes.
In the case of FIG. 1, with I.sub.B2 =1 mA: EQU P.sub.d =(0.3+1.2) 2+30.times.10.sup.-3 =3.03 W
In the case of FIG. 2, with I.sub.B1 =50 mA: EQU P.sub.d =0.8.times.2+30.times.50.times.10.sup.-3 +30.times.10.sup.-3 =3.13 W
One object of the present invention is to provide control means for a Darlington type or pseudo-Darlington type circuit which allows, with a high power voltage (for example 30 V) to reduce the dissipated power.
In order to achieve this object as well as certain others, the present invention provides a device for controling an output Darlington type circuit of an integrated circuit comprising two transistors, in which the first transistor has a first main electrode connected to a power supply terminal and a second main electrode connected to an earth terminal, a load being connected between one of these main electrodes and the corresponding terminal, the base of this first transistor being connected to a second main electrode of the second transistor of which the base receives a control current. In this control device, the first main electrode of the second transistor is connected to an auxiliary power supply (V..sub.AUX) so that EQU (V..sub.AUX) is substantially equal to V+V.sub.BE1 +V.sub.CE2.
where:
The auxiliary voltage V..sub.AUX is, according to one aspect of the present invention, obtained from the power supply source V.sub.CC through a continuous-continuous converter included in the integrated circuit comprising the Darlington type circuit.
The converter is preferably an inductance converter with accumulation capacity and discontinuous conduction of the "flyback" type comprising an inductance of which the charge duration is controled by an oscillating device and which is thereafter discharged into a capacitor.